Activation of the PI3K pathway is a frequent event in human tumors, promoting cell proliferation, survival, and resistance to chemotherapy and radiotherapy. (Shaw et al. Ras, PI(3)K and mTOR signalling controls tumour cell growth. Nature. 441 (2006), pp. 424-430; Verheijen et al. Phosphatidylinositol 3-kinase (PI3K) inhibitors as anticancer drugs. Drugs of the Future, 32 (2007), pp. 537-547.) Phosphoinositide 3-kinases (PI3-Ks) and mammalian target of rapamycin protein kinase (mTOR) are the key kinases in the PI3K signaling pathway.
PI3-Ks catalyze the synthesis of the phosphatidylinositol (PI) second messengers PI(3)P, PI(3,4)P2, and PI(3,4,5)P3 (PIP3). (Fruman et al., Phosphoinositide kinases, Annu. Rev. Biochem. 67 (1998), pp. 481-507; Knight et al., A Pharmacological Map of the PI3-K Family Defines a Role for p110α in Insulin Signaling, Cell 125 (2006), pp. 733-747.) In the appropriate cellular context, these three lipids control diverse physiological processes including cell growth, survival, differentiation, and chemotaxis. (Katso et al., Cellular function of phosphoinositide 3-kinases: implications for development, homeostasis, and cancer, Annu. Rev. Cell Dev. Biol. 17 (2001), pp. 615-675.) The PI3-K family comprises at least 15 different enzymes, sub-classified by structural homology, with distinct substrate specificities, expression patterns, and modes of regulation. The main PI3-kinase isoform in cancer is the Class I PI3-Kα, consisting of catalytic (p110α) and adapter (p85) subunits. (Stirdivant et al., Cloning and mutagenesis of the p110α subunit of human phosphoinositide 3′-hydroxykinase, Bioorg. Med. Chem. 5 (1997), pp. 65-74.) The 3-phosphorylated phospholipids (PIP3s) generated by PI3-Ks act as second messengers recruiting kinases with lipid binding domains (including plekstrin homology (PH) regions), such as AKT and phosphoinositide-dependent kinase-1 (PDK1). (Vivanco & Sawyers, The Phosphatidylinositol 3-Kinase—AKT Pathway In Human Cancer, Nature Reviews Cancer 2 (2002), pp. 489-501.) Binding of AKT to membrane PIP3s causes the translocation of AKT to the plasma membrane, bringing AKT into contact with PDK1, which is responsible for activating AKT. The tumor-suppressor phosphatase, PTEN, dephosphorylates PIP3 and therefore acts as a negative regulator of AKT activation. Functional loss of PTEN (the most commonly mutated tumour-suppressor gene in cancer after p53), oncogenic mutations in the PIK3CA gene encoding PI3-Kα, amplification of the PIK3CA gene and overexpression of AKT have been established in many malignancies. (see, for example, Samuels, et al., High frequency of mutations of the PIK3CA gene in human cancers, Science 304 (2004), p. 554; Broderick et al., Mutations in PIK3CA in anaplastic oligodendrogliomas, high-grade astrocytomas, and medulloblastomas, Cancer Research 64 (2004), pp. 5048-5050.) PI3-Kα is thus an attractive target for cancer drug development since such agents would be expected to inhibit proliferation and surmount resistance to cytotoxic agents in cancer cells.
mTOR is a serine/threonine kinase that controls the protein translation machinery and hence cell proliferation. (Faivre et al. Current development of mTOR inhibitors as anticancer agents. Nat Rev Drug Disc. 5 (2006), pp. 671-688; Rosner et al. The mTOR pathway and its role in human genetic disease. Mutation Research 3 (2008), pp. 284-292; Hall and Schmelzle Cell, TOR, a central controller of cell growth. Cell 103 (2000), pp 253-252.) mTOR is active in two complexes: mTORC1, which is sensitive to inhibition by the immune suppressant rapamycin, and mTORC2, which is not inhibited by rapamycin. (Sabatini et al. RAFT1 a mammalian protein that binds to FKBP12 in a rapamycin-dependent fashion and is homologous to yeast TORS. Cell 78 (1994), pp. 35-43); Sarbassov et al. Rictor, a novel binding partener of mTOR, defines a rapamycin-insensitive and raptor-independent pathway that regulates the cytoskeleton. Curr. Biol. 14 (2004), pp. 1296-1302.) The protein kinase activities of the two mTOR complexes can be regulated by signaling from growth factor receptors, via PI3K, or by the level of nutrients, particularly amino acids, available to the cell. In both cases mTOR regulation involves a protein complex comprised of TSC1 and TSC2. During growth factor signaling, RTKs activate PI3K, which in turn activates the protein kinases AKT and PDK1, via formation of PIP3. AKT can directly phosphorylate TSC2, which leads to inhibition of the GAP activity of the TSC1/TSC2 complex towards the GTPase Rheb. This in turn leads to activation of Rheb, which is thought to directly activate mTOR. The TSC1/TSC2 complex can also be regulated by PI3K-independent signals. Activation of the AMPK kinase in response to energy deprivation, such as low glucose or amino acids, leads to activation of the TSC1/TSC2 complex and downregulation of mTOR. Similar mechanisms account for suppression of mTOR by hypoxia and Wnt signaling. TSC1/TSC2 therefore serves as a point of integration of diverse cellular signals converging on mTOR regulation. Once activated, the mTORC1 complex phosphorylates two key substrates, 4EBP1 and S6 kinase. Phosphorylated 4EBP1 binds to ribosomal initiation factors and activated S6 kinase phosphorylates the ribosomal protein S6. The net result is the stimulation of cap-dependent translation and the synthesis of proteins that are required for entry into the DNA synthesis phase of the cell cycle. mTORC1 is therefore seen as a gatekeeper of cell cycle progression, integrating extracellular growth signals and energy status to decide whether the cell has an appropriate environment for proliferation. The mTORC2 complex does not regulate translation, but activates AKT by phosphorylation, leading to further mTOR activation as well as substrates such as BAD and FOXO that stimulate cell survival. mTOR-activated proteins promote several hallmarks of cancer such as cell growth and proliferation, angiogenesis, and bioenergetics. Since mTOR acts as a neoplastic switch that is frequently turned on by many mutations found in cancer, inhibition of mTOR offers a promising strategy for cancer therapy. Given that the mTOR pathway is deregulated in a number of cancers, it is anticipated that mTOR inhibitors will have broad therapeutic application across many tumor types.
Hence, in some tumors, targeting both PI3-Kα and mTOR may provide additional benefit compared with selectively targeting PI3-Kα. There is a need to provide new PI3-Kα inhibitors and/or dual PI3-Kα/mTOR inhibitors that are good drug candidates. They should be bioavailable, be metabolically stable and possess favorable pharmacokinetic properties.